In cellular communication, the most basic form of “handover” or “handoff” is when a phone call in progress is redirected from its current cell (called “source cell” or “serving cell”) to a new cell (called “target cell”). In terrestrial cellular radio networks, the source and the target cells may be served from two different cell sites or from a single cell site (in the latter case, the two cells are usually referred to as two “sectors” on that cell site). Such a handover, in which the source and the target cells are different cells (even if they are no the same cell site) is called an inter-cell handover. The purpose of an inter-cell handover is to maintain the call as the subscriber is moving out of the area covered by the source cell and entering the area of the target cell.
In a Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) network, the handover procedure starts with the measurement reporting of a handover event by the User Equipment (UE) to its serving evolved Node B (eNB or eNodeB) or Radio Base Station (RBS, or more simply “BS”). The UE periodically performs Downlink (DL) radio channel measurements based on the Reference Symbols (RS); namely, the UE can measure the Reference Symbols Received Power (RSRP) and the Reference Symbols Received Quality (RSRQ) from its serving cell as well as from the strongest adjacent cells in the system. If certain network-configured conditions are satisfied, the UE sends the corresponding measurement report (to its serving cell) indicating the triggered event. For example, when the handover (HO) algorithm is based on RSRP values, HO is triggered when the RSRP value from an adjacent cell is higher than the one from the serving cell by a network-specified number of decibels (dBs). In addition, the measurement report indicates the cell (referred to as “target cell”) to which the UE has to be handed over. Based on the measurement reports, the serving eNB (in UE's source cell) starts HO preparation. The HO preparation includes exchanging of signaling between serving and target eNBs and admission control of the UE in the target cell. The communication interface between the serving and target eNBs is called the “X2” interface, which may be used to carry out necessary HO-related signaling. Upon successful HO preparation, the HO decision is made (by the source cell) and, consequently, the HO Command will be sent to the UE (from the source eNB). The UE may then attempt to synchronize and access the target eNB to effectuate the handover.
One of the most time-consuming tasks in today's cellular networks is the optimization of handover relations. The Automatic Neighbor Relation (ANR) feature in LTE minimizes the need for the manual configuration of neighbor cell list for intra-frequency or inter-frequency handover. ANR automatically builds up and maintains a neighbor list used for handover. ANR adds neighbor relations to the serving cell's neighbor list when the UE measurement reports indicate that a possible new neighbor relationship has been identified. When this occurs, the serving eNB (or RBS) requests the UE to report the unique Cell Global Identity (CGI) of the potential neighbor cell. Using this information, the serving RBS/eNB automatically creates a neighbor relation between the serving cell and the new neighbor cell using the ANR procedure, thereby facilitating UE's handover to that neighbor cell. The ANR feature can be used together with manual optimization of neighbor lists, and ANR is also able to automatically remove neighbor cell relations which have not been used within a particular time period.
FIG. 1 illustrates an existing operational sequence as part of an Automatic Neighbor Relation (ANR) procedure in LTE to build a neighbor list at a serving cell 10 in a wireless system 12 for facilitating a future handover. For the sake of simplicity, only two cells—the serving cell 10 and a neighboring cell 14—are shown in FIG. 1 as part of the wireless system 12. It is understood that many such cells may form part of the system 12, and inter-cell handovers among those cells allow a UE (e.g., the UE 16) to “seamlessly” continue mobile communication throughout the system 12 and beyond. As mentioned earlier, the cells 10 and 14 may be part of different cell sites (not shown) or may belong to the same cell site (not shown). Furthermore, although cells 10 and 14 are illustrated far apart in FIG. 1 and although UE 16 is shown outside of both of these cells 10, 14, such illustration is for the sake of convenience and ease of discussion only. In the context of the handover related discussion with reference to FIG. 1, it is understood that the UE 16 may be physically present and operating (or registered) within the serving cell 10 or may be currently associated, with—i.e., under Radio Frequency (RF) coverage of—the serving cell 10 in some manner (e.g., through prior handover), but may need to be handed over to the neighboring/adjacent cell 14 if so determined by the eNB 18 associated with the serving cell 10 and providing RF coverage to the UE 16 within the serving cell 10. A different eNB 20 may be associated with the neighboring cell 14 to provide RF coverage over cell 14 and in its vicinity. The earlier-mentioned X2 communication interface between two eNBs 18, 20 is symbolically illustrated by dotted line 22. In the discussion below, the serving cell 10 may be interchangeably referred to as “Cell A” whereas the neighboring cell 14 may be interchangeably referred to as “Cell B.”
As shown in FIG. 1, the wireless system 12 may also include a Core Network (CN) 24 through which the eNBs 18, 20 may communicate with an Operations Support System for Radio and Core (OSS-RC) 26. Although not shown explicitly, it is noted here that each eNB 18, 20 may be connected to the CN 24 and the OSS-RC 26. Furthermore, the OSS-RC also may be connected to the CN 24 and may provide a proprietary (network operator-specific) platform for supervision, configuration, deployment and optimization of a mobile or cellular network (e.g., the wireless system 12), with features tailored to promote efficient working procedures in daily network operations. The OSS-RC 26 may provide full support for management of fault, performance, and network configuration, and may also provide a number of new applications that may be used in the trouble-shooting and network optimization stages. The CN 24 may be an Evolved Packet Core (EPC) in LTE. In FIG. 1, the block showing the CN 24 is shown dotted to indicate lack of any appreciable involvement of the CN 24 (or its component nodes) during the ANR procedure or subsequent handover operation.
The OSS-RC 26 may be implemented using a combination of hardware and/or software modules. Some exemplary modules in the OSS-RC 26 are shown in FIG. 1. As illustrated, the OSS-RC 26 may include, among others, a Domain Name Server (DNS) module 28, an OSS-RC Network Resource Model (ONRM) 29, and a Performance Management Support (PMS) module 30. As mentioned below, the DNS module 28 may store and provide on request Internet Protocol (IP) address of a cell or other related information for the cell in the wireless system 12. The ONRM module 29 may store data and messages (e.g., alarm or non-alarm messages) from various network elements or nodes (e.g., an eNB or RBS, or a node in the core network) in the wireless system 12 and make its content available to other OSS-RC modules or system components (not shown) for efficient management of network performance and handling of fault events. The PMS module 30 may provide an interface (e.g., a Graphical User Interface or GUI) to manage performance of various network elements or nodes (e.g., an RBS, a Radio Network Controller (RNC), a node in the core network, etc.) in the wireless system 12. The PMS module 30 may allow a network operator or other authorized third party to set up and administer collection of performance management data from different portions (e.g., a Radio Access Network (RAN) portion) of the overall cellular network (e.g., an LTE network).
In FIG. 1, arrow 32 illustrates the exemplary direction of handover of the UE 16—i.e., from the serving cell (Cell A) 10 to the target cell (Cell B) 14. It is understood that similar handover may be performed from Cell B to Cell A (or between any other pair of cells in the system 12), when necessary. However, for ease of discussion, only the handover from Cell A to Cell B is addressed here. The operational sequence—performed prior to the handover—for building a neighbor list (not shown) at the serving cell 10 using the ANR procedure is illustrated using arrows or other indicators associated with reference numerals 34 through 41.
FIG. 2 shows a flowchart 43 that provides details of each operation or step in the operational sequence 34-41 illustrated in FIG. 1. Thus, for the sake of convenience and ease of understanding, identical reference numerals are used in FIGS. 1 and 2 for the operational sequence 34-41, and FIGS. 1 and 2 are jointly discussed below to explain what steps are performed to build a neighbor list at the serving cell 10 using ANR. It is assumed here that the Physical Cell ID (PCI) of Cell A has been assigned a value of “3”, whereas the PCI of Cell B has been assigned a value of “5” as shown in FIG. 1. Similarly, it is also assumed that the Cell Global Identity (CGI) for Cell A has a value of “17,” whereas the CGI for Cell B has been assigned a value of “19.”
As is known, when the UE 16 is mobile, it may start receiving RF signals from the eNB 20 in the neighboring cell 14 along with the RF signals from its serving cell 10, especially when the UE 16 is in the vicinity of the neighboring cell 14. When the UE 16 receives RF signals from the neighboring cell (i.e., Cell B) 14, the UE 16 may initially report Cell B's signal measurements (as received by UE 16) to Cell A (as shown by arrow 34 in FIG. 1 and block 34 in FIG. 2). Such signal measurement may include, for example, the PCI of Cell B. Assuming Cell B is not on Cell A's current neighbor list, when Cell A receives PCI=5 (i.e., the PCI of Cell B) from the UE 16, Cell A may conclude that this PCI value is not known (as shown by circle 35 in FIG. 1 and block 35 in FIG. 2). Then, Cell A may order UE 16 to read CGI for Cell B (as shown by arrow 36 in FIG. 1 and block 36 in FIG. 2). In response, the UE 16 may read CGI broadcasted for Cell B and report that CGI to Cell A (as shown by arrow 37 in FIG. 1 and block 37 in FIG. 2). The UE 16 may also read and report RSRP and/or RSRQ values for the adjacent Cell B at step 37. Upon receiving Cell B's CGI, Cell A may automatically add (using ANR) Cell B as a neighbor cell in its neighbor list. Cell B's presence in Cell A's neighbor list would then allow Cell A to execute subsequent handover to Cell B, when needed.
Upon adding Cell B to its neighbor list, Cell A may check if X2 (i.e., communication interface) to Cell B is allowed (as shown by circle 38 in FIG. 1 and block 38 in FIG. 2). For example, there may be an “X2 Black List” stored in the source cell. If the target cell is not present in that “Black List,” then the source cell would assume that X2 to that target cell is allowed. If X2 with Cell B is allowed, Cell A may get IP address and CGI for Cell B from the DNS module 28 in the OSS-RC 26 (as shown by arrow 39 in FIG. 1 and block 39 in FIG. 2). Thereafter, Cell A may establish X2 with Cell B as indicated by arrow 40 in FIG. 1 and block 40 in FIG. 2. After X2 is established with Cell B, Cell A may update OSS-RC 26 with relevant observation data (as indicated by arrow 41 in FIG. 1 and block 41 in FIG. 2). The “observation data” may include all the configuration information needed to setup the handover. Updating the OSS-RC 26 with this data may help all future handovers (from Cell A to Cell B) because the system now has all the information needed for a future handover request and it need not ask the UEs to read CGI information (for Cell B) every time (e.g., as indicated at block 36 in FIG. 2). It is noted here that in case of LTE's Evolved Universal Terrestrial Radio Access Network (EUTRAN) air interface, the term “CGI” may be replaced with the term “ECGI” referring to EUTRAN Cell Global Identity. In any event, the terms “CGI” and “ECGI” may be interchangeably used hereinbelow, and the mention of one term and absence of the other in the relevant discussion below should not be construed to mean that the discussion does not apply to the non-mentioned term. In other words, reference to “CGI” also includes “ECGI” (if applicable), and vice versa. Furthermore, alternative embodiments may utilize other identifiers in addition to or as an alternative to CGIs and ECGIs. In one embodiment, the “observation data” may include, for example, the IP address for the other (target) cell, the PCI for the target cell or sector (the serving cell and the target cell may be referred to as “sectors” when they both belong to the same cell site), etc.
The operational sequence 34-41 discussed above is part of the ANR procedure at the serving cell (i.e., Cell A) 10. Upon conclusion of the operational sequence 34-41, eNB 18 in Cell A may perform handover of UE 16 to eNB 20 in Cell B when certain network-specified triggers for HO are present (as mentioned earlier).